A polyethylene terephthalate film is excellent in mechanical properties, heat resistance and dimensional stability at high temperatures and has heretofore been used as a base film for supporting light-gathering beads for the light-diffusing plate of a liquid crystal display.
In recent years, a further improvement in contrast in liquid crystal display and an increase in the size of a display panel are demanded. Such demands require an increase in the light quantity of a backlight light source which is a light source. However, the increase in the quantity of light makes the temperature of the liquid crystal display during operation higher than before. However, the conventional polyethylene terephthalate substrate has the problem that the quantity of light cannot be increased because the heat resistance of the substrate in the light-diffusing plate is not sufficient. Further, the polyester film for a light-diffusing plate has another problem that it is poor in luminance when it has a poor uniform thickness and contains a large amount of foreign matters.